JP2884855B2 - Temperature control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is used for a temperature control circuit of a heater. In particular, it relates to a temperature control circuit of a heater of a printer or the like.

[0002]

2. Description of the Related Art FIG. 7 is a block diagram of a conventional temperature control circuit.

Conventionally, as shown in FIG. 7, a temperature control circuit performs full-wave rectification on the voltage of an AC power supply 1 by a rectifying bridge 2 and turns on the rectified voltage of a thyristor 8 to thereby turn on the heating element 12. Is applied. At this time, the thermistor 11 is used as an element that converts temperature into impedance, and detects the temperature of the heating element 12 as the impedance of the thermistor 11. The thermistor 11 is connected in series with the resistor 9, the terminal of the resistor 9 not connected to the thermistor 11 is connected to the power supply terminal of the gate control circuit 4, and the terminal of the thermistor 11 not connected to the resistor 9 is It is connected to the common potential terminal of the gate control circuit 4 and divides the power supply voltage of the gate control circuit 4 by the resistor 9 and the thermistor 11 and outputs the divided voltage to the gate control circuit 4. The gate control circuit 4 outputs a voltage when the input voltage is equal to or higher than the threshold, and stops outputting the voltage when the input voltage is lower than the threshold.

Now, the temperature of the heating element 12 is set to thermistor 1
Detect at 1. It is assumed that the power supply voltage of the gate control circuit 4 is divided by the impedance of the thermistor 11 and the resistor 9 at this time, and the divided voltage is larger than the threshold value of the gate control circuit 4. In this case, the gate control circuit 4 outputs a voltage, applies a trigger to the trigger terminal of the thyristor 8, and turns on the thyristor 8. When the thyristor 8 is turned on, a rectified voltage is applied to the heating element 12.

When a voltage is applied to the heating element 12 and the temperature rises, the impedance of the thermistor 11 decreases, and the divided voltage of the thermistor 11 and the resistor 9 decreases. Control circuit 4
Stops the output of the voltage. Since the output voltage of the gate control circuit 4 is stopped, no trigger is applied to the thyristor 8, and the thyristor 8 is turned off when the current flowing through the thyristor 8 becomes equal to or less than the holding current.

The switching of the thyristor 8 is used to control the temperature of the heating element 12.

[0007]

However, in such a conventional temperature control circuit, when a trigger is applied to the gate terminal of the thyristor, the voltage value rectified by the rectifying bridge is 0 V.
The thyristor is turned “on” at any value of × 2 × the effective value of the AC voltage.
When a power supply of V is used, the thyristor may be turned on at a voltage of 50 V or more, and there is a disadvantage that noise is easily generated.

The present invention solves the above-mentioned drawbacks, and can set a low voltage after rectification when the thyristor is turned on, thereby reducing the noise generated when the thyristor is turned on. It is an object of the present invention to provide a temperature control circuit which can be used.

[0009]

According to the present invention, there is provided a rectifier having one terminal connected to a common potential for rectifying an AC voltage, a heating element having one terminal connected to a power supply terminal of the rectifier, and A thyristor inserted between the other terminal of the heating element and the common potential, a DC power supply having one terminal connected to the common potential, and a heating element connected to the DC power supply and having a temperature lower than a predetermined temperature. A gate control means for controlling the thyristor to be turned on at the time of turning on and turning off when the temperature exceeds a predetermined temperature. An overvoltage control means for controlling the gate control means to turn off the thyristor when the threshold value is exceeded is provided.

Further, according to the present invention, the gate control means includes:
When the control voltage connected to the DC power supply and input to the control terminal is equal to or higher than a predetermined threshold, the thyristor is turned on.
A first gate control circuit that is turned off when the voltage is less than a predetermined threshold, a first resistor having one terminal connected to a power supply terminal of the first gate control circuit, A temperature at which one terminal is connected to the other terminal of the resistor and the other terminal is connected to the common potential, and a divided voltage with the resistor is applied as the control voltage to decrease the impedance value when the temperature of the heating element rises. A detecting element, wherein the overvoltage control means is inserted between the DC power supply and a power supply terminal of the first gate control circuit, and when an output voltage of the rectification means becomes equal to or more than a predetermined threshold value. A first switch means to "turn off" may be included.

Further, according to the present invention, the gate control means turns on the thyristor when the control voltage of the control terminal is higher than a predetermined threshold value when the thyristor is connected to the DC power supply, and turns on the thyristor when the control voltage is lower than the predetermined threshold value. A first gate control circuit that is sometimes turned off, a first resistor having one terminal connected to a power supply terminal of the first gate control circuit, and one terminal connected to the other terminal of the first resistor. A temperature detecting element having a terminal connected to the other terminal connected to the common potential, a divided voltage with the first resistor being applied as the control voltage, and an impedance value decreasing when the temperature of the heating element rises. The overvoltage control means includes a second resistor inserted between a power supply terminal of the DC power supply and the first gate control circuit, and a power supply terminal of the first gate control circuit and the common potential. Connected in series between May include a second switching means for "on" when the output voltage of the input has been the third resistor and the rectifier means becomes less than a prescribed threshold.

Further, according to the present invention, the gate control means includes:
A first gate control circuit that is connected to the DC power supply and turns on the thyristor when the control voltage of the control terminal is equal to or higher than a predetermined threshold and turns off when the control voltage is lower than the predetermined threshold; A first resistor having one terminal connected to a power supply terminal of the first gate control circuit, one terminal connected to the other terminal of the first resistor, and the other terminal connected to the common potential; A temperature detecting element that provides a divided voltage with the first resistor as the control voltage and reduces the impedance value when the temperature of the heating element rises, wherein the overvoltage control means is provided between both terminals of the temperature detecting element. And a second switch means which is turned on when the output voltage of the rectifier becomes equal to or higher than a prescribed threshold value.

Further, according to the present invention, the gate control means turns on the thyristor when the control voltage of the control terminal is connected to the DC power supply and the control voltage is equal to or higher than a predetermined threshold, and the gate control means turns the thyristor below the predetermined threshold. A second gate control circuit that is sometimes `` off, '' a temperature detection element whose impedance value decreases when one of the terminals is connected to the power supply terminal of the second gate control circuit and the temperature of the heating element rises, A first resistor having one terminal connected to the other terminal of the temperature detecting element and the other terminal connected to the common potential and providing a divided voltage with the temperature detecting element as the control voltage; The control means may include first switch means inserted between both terminals of the temperature detection element and turned on when the output voltage of the rectification means becomes equal to or higher than a predetermined threshold value.

Further, according to the present invention, the gate control means includes:
A second gate control circuit that is connected to the DC power supply and turns on the thyristor when the control voltage of the control terminal is equal to or higher than a predetermined threshold and turns off when the control voltage is lower than the predetermined threshold; One terminal is connected to the power supply terminal of the second gate control circuit, and the impedance value decreases when the temperature of the heating element rises, and one terminal is connected to the other terminal of the temperature detection element. The other terminal is connected to the common potential and includes a first resistor for providing a divided voltage with the temperature detecting element as the control voltage, wherein the overvoltage control means includes the DC power supply and the second gate control circuit. A second resistor inserted between the power supply terminal of the second gate control circuit and a third resistor connected in series between the power supply terminal of the second gate control circuit and the common potential and the rectifier. Output voltage May include a second switching means for "on" when it is less than the prescribed threshold values.

Further, according to the present invention, the gate control means turns on the thyristor when the control voltage at the control terminal is higher than a predetermined threshold value and is connected to the DC power supply, and the gate control means turns on the thyristor when the control voltage is lower than the predetermined threshold value. A second gate control circuit that is sometimes `` off, '' a temperature detection element whose impedance value decreases when one of the terminals is connected to the power supply terminal of the second gate control circuit and the temperature of the heating element rises, A first resistor having one terminal connected to the other terminal of the temperature detecting element and the other terminal connected to the common potential and providing a divided voltage with the temperature detecting element as the control voltage; The control means is inserted between the first resistor and the control terminal of the second gate control circuit or the common potential, and is turned off when the output voltage of the rectification means becomes equal to or higher than a specified threshold value. " It may include a second switch means that.

[0016]

The overvoltage control means controls the gate control means to turn off the thyristor when the output voltage of the rectification means becomes equal to or higher than a specified threshold value.

As described above, the rectified voltage when the thyristor is turned on can be set low, and the noise generated when the thyristor is turned on can be reduced.

[0018]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a temperature control circuit according to a first embodiment of the present invention. In FIG. 1, a temperature control circuit includes an AC power supply 1 and a rectifier bridge 2 as one rectifier for rectifying an AC voltage with one terminal connected to a common potential, and a heat source having one terminal connected to a power terminal of the rectifier bridge 2. Element 12
A thyristor 8 inserted between the other terminal of the heating element 12 and the common potential; a DC power supply 3 having one terminal connected to the common potential;
And gate control means for performing control to turn on the thyristor 8 when the temperature is lower than a predetermined temperature and turn off when the temperature is higher than the predetermined temperature.

Here, the feature of the present invention is that when the output voltage of the rectifier bridge 2 exceeds a predetermined threshold value, the gate control means is controlled to turn off the thyristor 8. That is, an overvoltage control unit is provided.

The gate control means includes a DC power supply 3
A thyristor 8 that is turned on when the control voltage input to the control terminal is higher than a predetermined threshold value and that is turned off when the control voltage is lower than the predetermined threshold value; 4, a first resistor 9 having one terminal connected to the power supply terminal of the gate control circuit 4;
One terminal is connected to the other terminal, the other terminal is connected to a common potential, and a divided voltage with the resistor 9 is applied as the control voltage to decrease the impedance value when the temperature of the heating element 12 rises. The overvoltage control means is inserted between the DC power supply 3 and the power supply terminal of the gate control circuit 4 and is turned off when the output voltage of the rectifying bridge 2 becomes equal to or higher than a predetermined threshold value. The switch means 6 is included as the first switch means.

The operation of the temperature control circuit having such a configuration will be described.

In FIG. 1, an AC power supply 1 passes through a rectifying bridge 2, is full-wave rectified, and is applied to a heating element 12 by turning on a thyristor 8. The gate control circuit 4 outputs a voltage when the input voltage is equal to or higher than a predetermined threshold, and stops outputting the voltage when the input voltage is lower than the predetermined threshold.

The switch means 6 is turned off when the voltage of the AC power supply 1 rectified by the rectifier bridge 2 is equal to or higher than a predetermined threshold, and is turned on when the voltage is lower than the threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4 is divided by the resistance 9 and the impedance of the thermistor 11, and the voltage is input to the gate control circuit 4. When the divided voltage exceeds a predetermined threshold value of the gate control circuit 4, the gate control circuit 4
Performs an operation of outputting a voltage and applying a trigger to the thyristor 8.

At this time, if the voltage rectified by the rectifying bridge 2 is equal to or greater than a specified threshold value, the switch means 6 is turned off, and the voltage of the DC power supply 3 is not supplied to the gate control circuit 4. For this reason, the gate control circuit 4 does not apply a trigger to the thyristor 8 because the power supply voltage is not supplied, and therefore, the thyristor 8 does not turn on. When the rectified voltage becomes less than a predetermined threshold, the switch means 6 is turned “on” and supplies the voltage of the DC power supply 3 to the gate control circuit 4. At this time, the gate control circuit 4 applies a trigger to the thyristor 8, and the thyristor 8 is turned on.
I do.

FIG. 2 is a block diagram of a temperature control circuit according to a second embodiment of the present invention.

In FIG. 2, the AC power supply 1 passes through the rectifier bridge 2, is full-wave rectified, and is applied to the heating element 12 when the thyristor 8 is turned on. The gate control circuit 4 outputs a voltage when the input voltage is equal to or higher than a predetermined threshold, and stops outputting the voltage when the input voltage is lower than the predetermined threshold.

The switching means 6A is turned "ON" when the voltage of the AC power supply 1 rectified by the rectifying bridge 2 exceeds a predetermined threshold, and turned "OFF" when the voltage falls below the predetermined threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4 is divided by the resistance 9 and the impedance of the thermistor 11, and the voltage is input to the gate control circuit 4. When the divided voltage becomes equal to or higher than a predetermined threshold value of the gate control circuit 4, the gate control circuit 4 outputs a voltage and performs an operation of applying a trigger to the thyristor 8.

The switch means 6A is connected in series with the resistors 13 and 14, and the one in which the switch means 6A and the resistors 13 and 14 are connected in series is connected in parallel to both ends of the DC power supply 3. If the voltage rectified in step 2 is equal to or higher than a specified threshold, the switch 6A is turned on.
Then, the DC power supply 3 is short-circuited by the resistors 13 and 14, and the voltage divided by the resistors 13 and 14 is applied to the resistor 9 and the thermistor 11 connected in series. For this reason, the divided voltage divided by the resistor 9 and the thermistor 11 decreases, and the input voltage of the gate control circuit 4 decreases to be less than a predetermined threshold. When the input voltage of the gate control circuit 4 becomes lower than the predetermined threshold value, the gate control circuit 4 stops outputting the voltage, the trigger is not applied to the thyristor 8 until now, and the thyristor 8 is turned off. I do.

When the rectified voltage becomes less than the prescribed threshold value, the switch means 6A is turned off, the divided voltage divided by the thermistor 11 and the resistor 9 returns to a normal value, and the gate voltage is reduced. The control circuit 4 applies a trigger to the thyristor 8 so that the thyristor 8 is turned on.

FIG. 3 is a block diagram of a temperature control circuit according to a third embodiment of the present invention.

In FIG. 3, the AC power supply 1 passes through the rectifier bridge 2, is full-wave rectified, and is applied to the heating element 12 when the thyristor 8 is turned on. The gate control circuit 4 outputs a voltage when the input voltage is equal to or higher than a predetermined threshold, and stops outputting the voltage when the input voltage is lower than the predetermined threshold.

The switch means 6A is turned on when the voltage of the AC power supply 1 rectified by the rectifier bridge 2 is equal to or higher than a specified threshold, and is turned off when the voltage is lower than the specified threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4 is divided by the resistance 9 and the impedance of the thermistor 11, and the voltage is input to the gate control circuit 4. When the divided voltage becomes equal to or higher than a predetermined threshold value of the gate control circuit 4, the gate control circuit 4 outputs a voltage and performs an operation of applying a trigger to the thyristor 8.

The switch means 6A is connected in parallel to both ends of the thermistor 11. At this time, if the voltage rectified by the rectifier bridge 2 is equal to or greater than a specified threshold, the switch 6A is turned “on” and the thermistor 11 is short-circuited.
For this reason, the input voltage of the gate control circuit 4 becomes about 0 V, and the input voltage of the gate control circuit 4 becomes lower than a predetermined threshold. When the input voltage of the gate control circuit 4 becomes lower than a predetermined threshold value, the gate control circuit 4 stops outputting the voltage, the trigger which has been applied to the thyristor 8 is not applied, and the thyristor 8 is turned off. I do.

When the rectified voltage becomes less than the specified threshold value, the switch means 6A is turned off, the divided voltage divided by the thermistor 11 and the resistor 9 returns to a normal value, and the gate voltage is reduced. The control circuit 4 applies a trigger to the thyristor 8, and the thyristor 8 is turned on.

FIG. 4 is a block diagram of a temperature control circuit according to a fourth embodiment of the present invention.

In FIG. 4, the AC power supply 1 passes through the rectifier bridge 2, is full-wave rectified, and is applied to the heating element 12 when the thyristor 8 is turned on. The gate control circuit 4A outputs a voltage when the input voltage is lower than a predetermined threshold, and stops outputting the voltage when the input voltage is higher than the predetermined threshold.

The switch means 6 is turned off when the voltage of the AC power supply 1 rectified by the rectifier bridge 2 is equal to or higher than a predetermined threshold, and is turned on when the voltage is lower than the predetermined threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4A is divided by the resistance 9 and the impedance of the thermistor 11, and is input to the gate control circuit 4A. When the divided voltage falls below a predetermined threshold value of the gate control circuit 4A, the gate control circuit 4A performs an operation of outputting a voltage and applying a trigger to the thyristor 8.

At this time, if the voltage rectified by the rectification bridge 2 is less than the specified threshold, the switch means 6 is turned off, and the voltage of the DC power supply 3 is not supplied to the gate control circuit 4A. For this purpose, the gate control circuit 4A
An attempt is made to apply a trigger to the thyristor 8, but since no power supply voltage is supplied, no trigger can be applied to the thyristor 8. Therefore, the thyristor 8 does not turn on. When the rectified voltage becomes less than the specified threshold value, the switch means 6 is turned "on" and supplies the voltage of the DC power supply 3 to the gate control circuit 4A. At this time, the gate control circuit 4A
Applies a trigger to the thyristor 8, and the thyristor 8 is turned "on".

FIG. 5 is a block diagram of a temperature control circuit according to a fifth embodiment of the present invention.

In FIG. 5, the AC power supply 1 passes through the rectifier bridge 2, is full-wave rectified, and is applied to the heating element 12 when the thyristor 8 is turned on. The gate control circuit 4A outputs a voltage when the input voltage is lower than a predetermined threshold, and stops outputting the voltage when the input voltage is higher than the predetermined threshold.

The switch means 6A is turned on when the voltage of the AC power supply 1 rectified by the rectifier bridge 2 is equal to or higher than a specified threshold, and is turned off when the voltage is lower than the specified threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4A is divided by the resistance 9 and the impedance of the thermistor 11, and is input to the gate control circuit 4A. When the divided voltage becomes lower than the threshold value of the gate control circuit 4A, the gate control circuit 4A performs an operation of outputting a voltage and applying a trigger to the thyristor 8.

The switch means 6A is connected in series with the resistors 13 and 14, and the switch means 6A and the resistors 13 and 14 connected in series are connected in parallel to both ends of the DC power supply 3. If the rectified voltage is equal to or greater than the prescribed threshold value, the switch means 6A is turned on, and the DC power supply 3 is short-circuited by the resistors 13 and 14. To this end, the thyristor 8 is turned "off" by appropriately selecting the value of the resistor 14 such that the output voltage does not trigger the thyristor 8 even when the gate control circuit 6A attempts to output a voltage.

When the rectified voltage becomes equal to or less than the prescribed threshold value, the switch means 6A is turned off, and the normal voltage of the DC power supply 3 is applied to the gate control circuit 4A. , The thyristor 8 is turned on.

FIG. 6 is a block diagram of a temperature control circuit according to a sixth embodiment of the present invention.

In FIG. 6, the AC power supply 1 passes through the rectifier bridge 2, is full-wave rectified, and is applied to the heating element 12 when the thyristor 8 is turned “ON”. The gate control circuit 4A outputs a voltage when the input voltage is lower than a predetermined threshold, and stops outputting the voltage when the input voltage is higher than the predetermined threshold.

The switch means 6 is turned on when the voltage of the AC power supply 1 rectified by the rectifying bridge 2 is equal to or lower than a specified threshold, and is turned off when the voltage is equal to or higher than the specified threshold. The temperature of the heating element 12 is converted into impedance by the thermistor 11, the power supply voltage of the gate control circuit 4A is divided by the resistance 9 and the impedance of the thermistor 11, and is input to the gate control circuit 4A. When the divided voltage becomes lower than the threshold value of the gate control circuit 4A, the gate control circuit 4A performs an operation of outputting a voltage and applying a trigger to the thyristor 8.

The switch means 6 is connected in series with the thermistor 11. At this time, if the voltage rectified by the rectifying bridge 2 is equal to or greater than a predetermined threshold, the switching means 6
Is turned off, and the input voltage of the gate control circuit 4A is pulled up by the thermistor 11. For this reason, the input voltage of the gate control circuit 4A becomes substantially equal to the voltage of the DC power supply 3, and the input voltage of the gate control circuit 4A becomes higher than a predetermined threshold. When the input voltage of the gate control circuit 4A becomes equal to or higher than a predetermined threshold value, the output of the voltage is stopped, the trigger applied to the thyristor 8 is not applied, and the thyristor 8 is turned off.

When the rectified voltage becomes less than the prescribed threshold value, the switch means 6 is turned on, the divided voltage divided by the thermistor 11 and the resistor 9 returns to a normal value, and the gate voltage is reduced. The control circuit 4A applies a trigger to the thyristor 8, and the thyristor 8 is turned on.

[0049]

As described above, according to the present invention, the rectified voltage when the thyristor is turned on can be set low, and the noise generated when the thyristor is turned on can be reduced. There is an excellent effect that can be done.

[Brief description of the drawings]

FIG. 1 is a block diagram of a temperature control circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram of a temperature control circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram of a temperature control circuit according to a third embodiment of the present invention.

FIG. 4 is a block diagram of a temperature control circuit according to a fourth embodiment of the present invention.

FIG. 5 is a block diagram of a temperature control circuit according to a fifth embodiment of the present invention.

FIG. 6 is a block diagram of a temperature control circuit according to a sixth embodiment of the present invention.

FIG. 7 is a block diagram of a conventional temperature control circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 AC power supply 2 Rectifier bridge 3 DC power supply 4, 4A Gate control circuit 6, 6A Switch means 8 Thyristor 9, 13, 14 Resistance 11 Thermistor 12 Heating element

Claims (7)

(57) [Claims] 1. A rectifier having one terminal connected to a common potential to rectify an AC voltage, a heating element having one terminal connected to a power supply terminal of the rectifier, and the other terminal of the heating element being connected to the other terminal. A thyristor inserted between the thyristor and the common potential, a DC power supply having one terminal connected to the common potential, and the thyristor being turned on when the heating element is connected to the DC power supply and the heating element is lower than a predetermined temperature. And a gate control means for performing control to turn off when the temperature exceeds a predetermined temperature, wherein when the output voltage of the rectification means exceeds a predetermined threshold value, A temperature control circuit comprising overvoltage control means for causing said gate control means to perform control to turn off said thyristor. 2. The gate control means turns on the thyristor when a control voltage connected to the DC power supply and input to a control terminal is equal to or higher than a predetermined threshold, and turns on when the control voltage is lower than the predetermined threshold. A first gate control circuit that is "off";
A first resistor having one terminal connected to a power supply terminal of the first gate control circuit, one terminal connected to the other terminal of the first resistor, and the other terminal connected to the common potential; A temperature detecting element for providing a divided voltage with the resistor as the control voltage and for decreasing the impedance value when the temperature of the heating element rises, the overvoltage control means including the DC power supply and the first gate control circuit; "Off" when the output voltage of the rectifier is inserted between the
2. The temperature control circuit according to claim 1, further comprising a first switch means. 3. The gate control means turns on the thyristor when the control voltage of the control terminal connected to the DC power supply is equal to or higher than a predetermined threshold, and turns off when the control voltage is lower than the predetermined threshold. A first gate control circuit, a first resistor having one terminal connected to a power supply terminal of the first gate control circuit, and one terminal connected to the other terminal of the first resistor. A temperature detecting element that has the other terminal connected to the common potential and provides a divided voltage with the first resistor as the control voltage, the impedance value of which decreases when the temperature of the heating element rises; The means includes a second resistor inserted between a power supply terminal of the DC power supply and the first gate control circuit, and a serial connection between a power supply terminal of the first gate control circuit and the common potential. Connected and inserted third Temperature control circuit according to claim 1 further comprising a second switching means for "on" when the output voltage of the anti and the rectifier means becomes less than a prescribed threshold. 4. The gate control means turns on the thyristor when the control voltage of the control terminal connected to the DC power supply is equal to or higher than a predetermined threshold, and turns off when the control voltage is lower than the predetermined threshold. A first gate control circuit, a first resistor having one terminal connected to a power supply terminal of the first gate control circuit, and one terminal connected to the other terminal of the first resistor. A temperature detecting element that has the other terminal connected to the common potential and provides a divided voltage with the first resistor as the control voltage, the impedance value of which decreases when the temperature of the heating element rises; 2. A temperature as claimed in claim 1, wherein said means includes a second switch inserted between both terminals of said temperature detecting element and turned on when an output voltage of said rectifier becomes equal to or higher than a prescribed threshold value. Control circuit. 5. The gate control means is connected to the DC power supply and turns on the thyristor when a control voltage at a control terminal is equal to or higher than a predetermined threshold, and turns off when the control voltage is lower than the predetermined threshold. A second gate control circuit, a temperature detection element having one terminal connected to a power supply terminal of the second gate control circuit, the impedance value of which decreases when the temperature of the heating element rises, and the temperature detection element A first resistor connected to one terminal to the other terminal and the other terminal connected to the common potential and providing a divided voltage with the temperature detection element as the control voltage, the overvoltage control means includes: 2. The temperature control circuit according to claim 1, further comprising: first switch means inserted between both terminals of said temperature detection element and turned on when an output voltage of said rectification means exceeds a specified threshold value. 6. The gate control means is connected to the DC power supply and turns on the thyristor when a control voltage at a control terminal is equal to or higher than a predetermined threshold, and turns off when the control voltage is lower than the predetermined threshold. A second gate control circuit, a temperature detection element having one terminal connected to a power supply terminal of the second gate control circuit, the impedance value of which decreases when the temperature of the heating element rises, and the temperature detection element A first resistor connected to one terminal to the other terminal and the other terminal connected to the common potential and providing a divided voltage with the temperature detection element as the control voltage, the overvoltage control means includes: A second resistor inserted between a power supply terminal of the DC power supply and the second gate control circuit, and connected in series between a power supply terminal of the second gate control circuit and the common potential. Inserted number Temperature control circuit according to claim 1 further comprising a second switch means the output voltage of the resistor and the rectifier means is "on" when it is less than a prescribed threshold. 7. The gate control means is connected to the DC power supply and turns on the thyristor when a control voltage at a control terminal is equal to or higher than a predetermined threshold, and turns off when the control voltage is lower than the predetermined threshold. A second gate control circuit, a temperature detection element having one terminal connected to a power supply terminal of the second gate control circuit, the impedance value of which decreases when the temperature of the heating element rises, and the temperature detection element A first resistor connected to one terminal to the other terminal and the other terminal connected to the common potential and providing a divided voltage with the temperature detection element as the control voltage, the overvoltage control means includes: The second, which is inserted between the first resistor and the control terminal of the second gate control circuit or the common potential, and is turned off when the output voltage of the rectifier becomes equal to or higher than a predetermined threshold value The switch Temperature control circuit according to claim 1 further comprising a means.